1. Field of the Invention
This invention relates to a mold for use in molding foaming mixture and the means to vent this mold as the foaming mixture fills the mold. More specifically, this invention relates to enhanced foam containment vents for a mold and a structure for such vents which rapidly obturate, significantly reduce the loss of material through the vent, are self-cleaning, and significantly reduce the amount of collapse of the foaming material in the region of the vent.
2. Description of the Prior Art
In the manufacture of molded foamed products, such as high resilience (HR) polyurethane foam products, for example, in automotive seating and other products, it is common practice to utilize molds which have air vents, e.g., molds with vented lids. The venting provides for the displacement of air out of the mold as the foaming mass expands to fill the various intricacies of the mold. This venting is necessary in order to preclude air entrapment in the molded product and also to prevent the incomplete filling of the mold by the expanding foam thereby resulting in a defective part. Consequently, it is necessary to have some form of venting as a part of the mold. It is general practice to locate vents in a mold in a manner such that as the foam mass expands the foaming polymer can readily push the air towards the vents and thus expand to the vent areas. In actual practice the expanding foam will expand to the area of the vent and will then in most cases expand into and through the vent forming a small bun of material on the vent surface on the exterior of the mold. In this way the molder is assured that the mold has been filled with the expanding foam. However, all of the foam that exudes through one or more of the vents is essentially wasted material. In the ideal situation the foam would completely fill the intricacies of the mold and expand to the area of each of the vents, but would not expand into and through the vent.
There is additionally the problem that when the foam expands into the vents that the vents will become clogged. In that instance prior to the mold being reused in another cycle to produce a product the vents must be cleaned in order to assure that they will work properly in the next molding cycle. This cleaning of the mold vents requires additional personnel for the molding operation and further increases the cycle time for producing the molded products.
In practice in a polyurethane foam system, as the expanding urethane reaches the vents there are still residual expanding forces existent resulting from the water-isocyanate blowing reaction which cause a portion of the still reacting (i.e., polymerizing) urethane mass to extrude through the vent holes forming a "bun" or "mushroom" of foam on the back of the mold lid. These vent exudates represent lost material and in some applications can represent material losses of 5-7%. In addition, as final mold fill out occurs in the latter stages of the ongoing urethane polymer growth reaction the foaming mass at the time of exudation is approaching the gelatin stage where mobility is lost. Further movement of the expanded mass as would result from blowing reaction forces causing or forcing the near gelled polyurethane to flow towards and through the vents result in collapse of foam in the areas adjacent to the vents. Typical vents range in size from .about.1/16" up to .about.1/4" in diameter and if plugged with urethane from a previous molding and not properly cleaned will be the source of a defect (air entrapment) in the next molding. Typical HR mold venting therefore has three disadvantages: (1) material loss as vent exudate, (2) foam collapse due to late movement, and (3) a need for regular cleaning and visual inspection prior to each molding.
The ideal mold venting mechanism would allow for total air displacement from the mold and at the instant of final mold fill-out the vent would be closed or sealed thereby precluding all exudation. In the normal mold venting technique currently employed in the polyurethane foam molding industry, vent obturation occurs only when the urethane system passes through and cures within the vent so as to form an in situ plug thereby preventing further foam compound exudation. In most cases this occurs only after significant material has already escaped through the vent.
It is a commonly employed technique to use vent diameter as a variable to control flow of the expanding urethane foam within a mold. A larger diameter vent will allow a greater air volume to pass through at a given pressure differential than will a smaller vent, therefore in-mold froth flow will be biased towards a larger vent. In addition a smaller vent will not only be more restrictive to air flow but will obturate with foam more quickly once foam reaches it and will exhibit less exudate. The greater exudate through a larger diameter vent and its later obturation will also be more likely to exhibit foam instability or collapse in the areas adjacent to the vent. This instability or collapse tendency is a function of the higher shear regimen (greater mass flow) and the fact that the foam is being forced to move in the later polymer growth reaction stages where fluidity or effective system mobility is being rapidly lost.
A great deal of effort has been expended in trying to resolve these problems. A known technique in the industry to limit foam extrusion through air bleed vents is to utilize vents covered with porous tape or in some cases fabric. The paper tape or the fabric will allow air to pass through but will quickly plug when the urethane reaches the porous element. While this system works well, it requires that each vent be manually served before each molding. Some molds may have 25 or more vents making this technique impractical except for certain specialized applications.
The driving force for urethane foam exudation through open vents is the differential pressure across the vent resulting from the CO.sub.2 generated by the reaction of the isocyanate and water and may be augmented by an auxiliary blowing agent such as a fluorocarbon. Exudation ceases only when either the vent becomes effectively sealed or the pressure internal to the mold is effectively vented, i.e., equilibrated with atmospheric pressure. It appears feasible that pressure equilibration could be utilized as a means of limiting exudation if at the proper time, i.e., time of complete mold fill out, the pressure external to the mold vent could be balanced with that internal to the mold. One technique of achieving this is to have variable volume chambers on the normally atmospheric side of each vent. Into these chambers air displaced from the mold by the expanding urethane is collected and the pressure continues to rise until it becomes equilibrated with that internal to the mold. Adjustment of the volume of such chambers would allow this equilibration to coincide with complete mold fill out thereby effectively precluding any exudation. A system utilizing a concept similar to this is disclosed in Japanese Patent Publication 83/143716.
The negative factors associated with this technique are an expected inability to handle day to day changes in line parameters which can effectively change the normal in-mold pressures. Examples of such causatives are mold temperatures, reactant stream temperature, fill weight variances, mold leakage, mixing efficiency and compound raw material variations that may effect reactivity (specifically small changes in formulation water). In addition, this technique would appear to be practically limited to simple molds requiring a small number of vents.
In Japanese Patent Publication 83/148458 there is disclosed the use of a vent which has vent holes in the form of slits. This contrasts with the usual vent hole which is circular in shape. The objective in the use of a slit form for a vent hole is to provide for a firm fitting in the opening that carries the vent and to maintain a good and secure fit during the thermal expansion and contraction of the mold.
Japanese Patent Publication 83/143716 (Kokai, Sho 55-17511/80) discloses a box-shaped air storage chamber that is provided on the top of a vertical mold and which enables the molding of foams without clogging the fine gas vent holes. This reference reports that the use of this air storage chamber precludes any significant clogging of the fine gas vent holes with the foam.
In contrast, U.S. Pat. No. 4,555,087 discloses a method of preventing the clogging of the vent openings by means of heating the vent openings. The objective in this patent is to provide sufficient heat at the area of the vent opening so that the foam will rapidly set in this area and not expand into the vent and thus clog the vent.
French Patent Application No. 2513933 discloses a method for making porous mold tools. These tools are made from casting resins which have arrayed there-through a number of pins or rods. After the resin has been solidified the pins or rods are removed thereby providing a number of vents that will be used when the mold tool is utilized for the production of particular products. The resulting vents are long and are difficult to clean when they become clogged.
The problem of foam extruding into and through the air vent openings of a mold was also addressed in German Pat. No. 1704566. It is noted in this patent that a considerable amount of product can be lost due to foam exuding through vent openings This patent suggests the use of a sieve or similar covering on the vent openings to prevent loss of foam and discloses that the sieve can be made of tissue or of a perforated plate having openings of about 0.5 to 1 mm. This patent also notes that any foam residues which adhere to the sieves can be removed easily by brushing or some similar method. The German patent states that the vent openings in the wall of the mold should not be less than 3 millimeters in diameter and they can be of a larger size such as 10 millimeters in diameter or greater. The problem with regard to the technique of the German patent is that there is a disproportionate decrease in the cross-sectional area of the vent opening, i.e., the size of the effective opening (through which gas can flow) of the smaller vent openings is reduced to a greater extent than that of the larger vent openings, resulting in disruption of the direction and rate of flow of foaming material in the mold which can result in too much material flowing to the larger vent openings and not enough flowing to the smaller vent openings, leaving voids in the vicinity of the smaller vent openings.
German Pat. No. 3026238 discloses a technique for preventing a molding composition from exuding through vent openings by covering over the vent openings with a very thin flexible foil. The flexible foil permits some or most of the air to escape through the vents but precludes the passage of the expanding elastomer through the air vents.
In contrast, in U.S. Pat. No. 3,880,557 there is disclosed a technique of capping off the air vents as the molded material displaces air along the length of a cavity. By the use of more intricate controls the various air vents can be closed when the material being molded passes into the region of an air vent. This provides a fairly complex technique in trying to resolve the above-mentioned problems.
Another technique that has been used is shown in U.S. Pat. No. 3,844,523 which discloses the utilization of a breathable release sheet to vent the air from a mold as a product is being molded. In this way the air in the mold is released over a larger area.
Japanese Patent Publication No. 77/1654 attempts to control the escape of air through vents of a mold by using weighted covers to avoid overflow of expanding material from the mold. Japanese Patent Publication 78/35532 discloses the use of glass wool to cover mold vents.
There are yet other techniques that have been attempted with regard to solving the above-mentioned problems. In prior attempts to be assured that all of the intricacies of the mold are filled with the foaming product, no convenient techniques have been brought forward where the foaming material will not expand into the vents, and further will provide for a more uniform product throughout especially in the region of the vents. The objective of filling all of the intricacies of a mold appears to be directly at odds with the objective of trying to keep the foaming mixture from exuding into and through the vent holes.